Device for measuring acceleration

ABSTRACT

A device for acceleration measurement is proposed, in which measuring signals from a two-dimensional sensor from different measuring directions are combined with one another and then jointly evaluated in an evaluation circuit. This evaluation is carried out, for example, using a threshold-value discriminator, in order to perform a plausibility check for a crash detection.

BACKGROUND INFORMATION

[0001] The present invention is directed to a device for accelerationmeasurement according to the species defined in the independent claim.

[0002] From published patent application WO 98/58264, it is alreadyknown to use a two-dimensional sensor element for measuringacceleration, the measuring signals being evaluated for the twodifferent measuring directions by separate demodulators. For thatreason, starting out from the related art, it is the underlyingobjective of the present invention to devise a simplified evaluation ofthe measuring signals from a two-dimensional sensor element.

Summary of the Invention

[0003] In contrast, the device according to the present invention formeasuring acceleration, having the features of the independent claim,has the advantage that a combination of the measuring signals from thevarious measuring directions is evaluated in one circuit, jointly andsimultaneously. As a result, the outlay is considerably reduced.Particularly, in applications involving motor vehicle technology and, inthis context, in crash sensing, crash plausibility is able to be simplydetermined when working with front-end-impact or side-impact sensors. Inthe case of front-end-impact and/or side-impact sensing, besides onlyone or a plurality of sensor(s) for the actual impact sensing, there isusually one additional, independent sensor for determining theplausibility of a crash-sensing signal. In this manner, the actual crashsensors are monitored for correct functioning.

[0004] Advantageous improvements of the device for accelerationmeasurement indicated in the independent claim are rendered possible bythe measures and further refinements delineated in the dependent claims.

[0005] It is particularly beneficial that for both measuring devices,clock-pulse generators are provided on the two-dimensional sensorelement, and that the ratio of the frequency supplied by each of the twoclock-pulse generators is even-numbered. In this manner, in an evenclock cycle, the signals are obtained in phase, and the odd-numbered inantiphase.

[0006] It is furthermore beneficial that the evaluation circuit of thedevice according to the present invention has a threshold-valuediscriminator, so that the combination of the measuring signals from thevarious measuring directions is compared to a threshold value determinedfrom experiments and simulations, in order to decide, when a triggeringevent is detected by the crash sensors, and its plausibility is to bedetermined, whether it is a question of an impact or not.

[0007] A further optimization is made possible by a combination with anadditional acceleration sensor.

BRIEF DESCRIPTION OF THE DRAWING

[0008] Exemplary embodiments of the present invention are illustrated inthe drawing and are explained in detail in the following description.The figures show:

[0009]FIG. 1 a principle of operation of a two-dimensional sensormembrane;

[0010]FIG. 2 a block diagram of the device according to the presentinvention;

[0011]FIG. 3 a second block diagram of the device according to thepresent invention; and

[0012]FIG. 4 the time relationships of the various measuring voltagesfor an even-numbered frequency ratio, selected here to equal two.

DETAILED DESCRIPTION

[0013] Acceleration sensors are being increasingly used in the motorvehicle industry. Such acceleration sensors are being manufactured moreand more frequently from semiconductors, using micromechanics. In thiscontext, a membrane is used as an acceleration-sensitive sensor element.Acceleration may be picked up in a surface in the X- and Y-directionsusing such a membrane.

[0014] In accordance with the present invention, such measuring signalsare combined from measuring directions which are perpendicular to oneanother, and evaluated jointly and simultaneously in an evaluationcircuit.

[0015]FIG. 1 depicts the functioning method of a two-dimensionalmembrane. A membrane CM has two fingers 4 and 5, which, in aninterdigital structure having external circuit elements, form twocapacitors. Interdigital finger 4 forms a first capacitor CHY with thetop electrode, and a second capacitor CLY with the bottom electrode. Aclock signal having frequency FY is transmitted to the electrodes.

[0016] Interdigital finger 5 forms a first capacitor CHX with the leftelectrode, and a second capacitor CLX with a right electrode. Here, aswell, a clock-pulse signal having a frequency FX is transmitted to theseries connection of capacitors CHX and CLX. Therefore, interdigitalfingers 4 and 5 provide a series connection of two capacitors. In aneutral position, interdigital fingers 4 and 5 are at neutral values inthe middle between the particular electrodes. This middle ischaracterized by neutral values YO and XO, YO being the neutral valuefor interdigital finger 4, and XO the neutral value for interdigitalfinger 5.

[0017] Depending on which direction interdigital fingers 4 and 5 aredeflected, the capacitances of capacitors CHY, CLY, CHX and CLX change.Since the distance between two electrodes is inversely proportional tothe capacitance between the electrodes, in response to the movement ofinterdigital finger 4, capacitance CHY will increase in the direction ofthe top electrode, while the capacitance of capacitor CLY decreases,since the distance to the lower electrode increases to the same degree.This consideration applies likewise to interdigital finger 5 and tocapacitors CHX and CLX.

[0018]FIG. 2 shows the device for acceleration measurement according tothe present invention as a block diagram. In this context, an equivalentcircuit diagram is provided where capacitors CHX, CLX, CHY and CLY shownin FIG. 1 are transposed to a circuit having a shared connection 6, fromwhich a combination and, in fact, a summation of the measured signals,these being here tapped off voltages UHX, ULX, UHY and ULY, areevaluated jointly and simultaneously by an evaluation circuit 1.Initially, the combined measured signals are amplified by an amplifierV, here a charge amplifier having a basic-value filter being used.However, other amplifier circuits and amplifier types may also be usedhere. After that, a rectifier network G follows which maps the combinedsignal in terms of absolute value.

[0019] Since, as shown further below, depending on the clock cycle, themeasuring voltages are added once, and, on the other hand, a subtractionof the measuring voltages is carried out, an amount of X+Y is present,on the one hand, and an amount of X-Y, on the other hand. The thusrectified, combined signal is then compared in a threshold-valuediscriminator S to a predefined threshold value to determine whetherthis value was exceeded or not. Thus, the side and front-impactdetection, respectively, recognize whether a front or a side impact ispresent. Therefore, a more accurate sensor is rendered plausible. Thisevaluation signal, which therefore signifies whether there is an impactor not, is then present as output signal A. It is possible that morethan one sensor signal is determined to be plausible in this manner.

[0020] In FIG. 3, a second block diagram is depicted which representsthe device according to the present invention for accelerationmeasurement. Besides the combined measuring signal from two-dimensionalsensor 2, evaluation circuit 1 also receives a further accelerationsignal, in this case from an acceleration sensor in the direction oftravel.

[0021]FIG. 4 illustrates the time relationships of the various voltages,measured across the capacitors. The voltages across capacitors CHX andCLX, here UHX and ULX, due to clock frequency FX, are clocked with halfof the frequency at which voltages UHY and ULY are clocked. For thatreason, four sections a, b, c and d may be distinguished here. Forsections a and d, it holds that:$U_{M} \sim {f\left( \frac{C_{HX} \cdot C_{HY}}{C_{LX} \cdot C_{LY}} \right)} \sim {f\left( {\frac{x_{0} - x}{x_{0} + x} \cdot \frac{y_{0} - y}{y_{0} + y}} \right)} \approx {f\left( {x + y} \right)}$

[0022] while for sections b and c, it holds that:$U_{M} \sim {f\left( \frac{C_{HX} \cdot C_{LY}}{C_{LX} \cdot C_{HY}} \right)} \sim {f\left( {\frac{x_{0} - x}{x_{0} + x} \cdot \frac{y_{0} + y}{y_{0} - y}} \right)} \approx {f\left( {x - y} \right)}$

[0023] here, f denotes a functional relationship. This explains whydifferent frequencies are used for the two clock-pulse generators.

What is claimed is:
 1. A device for acceleration measurement, the devicehaving a two-dimensional sensor element (CM), an evaluation circuit (1)being provided for processing signals from the two-dimensional sensorelement (CM), wherein the evaluation circuit (1) evaluates a combinationof the signals from accelerations in a first and a second direction, thefirst and the second direction forming a right angle.
 2. The device asrecited in claim 1, wherein a clock-pulse generator is provided for eachof the first and second directions, an even-numbered factor, preferablytwo, existing between the particular clock frequencies.
 3. The device asrecited in claim 1 or 2, wherein a threshold-value discriminator (S) isprovided in the evaluation circuit (1) to compare the combination to athreshold value.
 4. The device as recited in claim 1, 2, or 3, whereinthe device has a further acceleration sensor (3).
 5. The device asrecited in one of the preceding claims, wherein an output signal of thethreshold-value discriminator (S) is used for determining plausibilityfor a crash detection.